I am planning to semi-permanently install my T-max 160 compressor in the loadbin, with a direct connection to the dual battery in the engine bay. There is an existing (unfused) cable running to the rear bumber with a brad harrison plug.
The compressor spec says it will draw 45 amps max. I am worried about a short circuit at the plug with a deep water crossing and would like to know if a fuse will prevent this and if so, what size fuse to install?
With a bit of work I can reroute the plug to inside the loadbin (under an alu-cab). What size fuse should I install in this case?
Any advice will be appreciated, my electronic knowledge is scary.

So if I understand you correctly, you want to protect the compressor as well as the cable running to the bumper?

The principle is always (no matter what voltage or whether AC/DC), that the protection device's current rating should be lower than that of the conductor.

As for the short ciruit in water, I would not be too concerned unless it is salt water. Just seal the cable entry side of it and get something to plug the business end with.

My suggestion is to put a main fuse on the second battery to protect it from overcurrent, then from there split into 2 circuits each with its' own fuse....50A for the compressor if it doesn't have it's own fuse and maybe 50A on the cable going to the bumper, but the latter is really optional IMO. It all depends on your application at the rear bumper.